1. Field of the Invention
The present application generally relates to outboard motors, and more particularly to outboard motors with lubrication systems.
2. Description of the Related Art
Watercraft vehicles, such as boats, are often powered by an outboard motor having an internal combustion engine. Japanese Patent Publication No. 2003-003844 and Japanese Patent Publication No. 8-268390 disclose outboard motors that include a generally vertically extending outer casing that can be mounted on a watercraft by a bracket assembly. A power transmitting shaft often extends generally vertically through the outer casing between an internal combustion engine and a drive gear. The drive gear engages a driven gear which is fixedly attached to a propeller shaft. A lubricant can be used to lubricate the drive gear and driven gear.
When the engine operates, the engine output is often transmitted to the propeller shaft through the power transmitting shaft, the drive gear, and then the driven gear. A propeller rotates together with the propeller shaft. The watercraft thus can move forward or backward depending on the direction of rotation of the propeller shaft. The teeth of the drive gear and corresponding teeth of the driven gear are lubricated by the lubricant to reduce wearing.
Japanese Patent Publication No. 8-268390 discloses an outboard motor having an oil pump that draws lubricant in an oil bath through a suction passage. The lubricant then flows towards the engine. The drive and driven gears are completely soaked by the lubricant in the oil bath. Unfortunately, when both of the gears rotate during engine operation, each tooth of one gear leaves from a space between two teeth of the other gear, and the space is immediately filled with lubricant. On the other hand, another tooth of the former gear enters another space between two teeth of the latter gear to push out lubricant which was disposed in the space between the two teeth. Such pumping caused by the rotating gears causes a significant energy loss. These energy losses undesirably reduce the output of the outboard motor.
Additionally, because of the pumping action generated at the engaging portions of the drive and driven gears, the temperature of the lubricant rises. The elevated temperature causes a reduction of viscosity of the lubricant. The film of lubricant thus becomes relatively thin at the engaging portions of the drive and driven gears. As such, the lubrication at the engaging portions of the gears can be insufficient to effectively prevent wearing. The heating can also accelerate the deterioration of the lubricant to further reduce the effectiveness of the lubricant.
If the lubrication becomes insufficient, or the lubricant deteriorates, the surfaces of the teeth of the gears can have pitching, scuffing, spalling, abrasion, and/or the free ends of the gear teeth can have chipping. Thus, the gears can be damaged due to improper lubrication.